IIRC, Busta said all his NEFs measured .502 land to land. The MMP HPH-12 or MMP Short black might give a better fit if yours is the same.

I don't have anything to measure my NEF bore, but I use crush rib sabots and 300gr XTPS and also FPB's with great results like your number 8 target. don't forget to get the lowes number 60 O ring, I replace mine after 25 shots or so.

Semi , how tight is your forearm screw? Also, are the original scratches you said you had at the muzzle end of the barrel still there or did they polish out.

The MU plug is sweet for sure ,Busta did a bunch of testing on it and it shows. He is the guru on them , if you have questions I am sure he can help.
Steve

Forend screw is quite tight and the sides of the forend are very tight against the sides of the barrel. Scratches are still there. They don't seem to be an issue and don't hold fouling when I clean the bore.

One thing I forgot to do in this session that I wanted to do. That was shoot a target with the best load I found, but with the forend off. Oh well, next time.

That was a fine report on your NEF muzzy Semi. Its amazing how those flyers come to be - always a mystery. Maybe it could be the dragon putting a curse on your other muzzys because you aren't giving him enough exercise???
BTW - I would love a day of 52* - I haven't seen anything above the 40s for weeks on end here.